A nitride semiconductor device has been actively developed as a semiconductor device having a high withstand voltage and a high output power by using its features (such as, a high saturation electron velocity and a wide band gap). As the nitride semiconductor device, many field effect transistors, in particular, high electron mobility transistors (HEMTs), have been reported. In particular, an AlGaN/GaN HEMT using GaN as an electron transit layer and AlGaN as an electron supply layer has drawn attention. In the AlGaN/GaN HEMT, distortion resulting from the difference in lattice constant between GaN and AlGaN is generated in AlGaN. Accordingly, by piezo polarization generated by the above distortion and spontaneous polarization of AlGaN, a two-dimensional electron gas (2DEG) at a high concentration is obtained. Accordingly, a high withstand voltage and a high output power are realizable.
As documents of related techniques, for example, there are Japanese Unexamined Patent Application Publication Nos. 2007-19309, 2009-76845 and 2004-335960.